The present invention relates generally to electronic ballasts for powering discharge lamps. More particularly, the present invention relates to electronic ballasts and associated methods for operating discharge lamps connected in parallel and capable of providing filament heating during a lamp dimming operation.
Electronic ballasts with dimming capability are becoming increasingly popular, due at least in part to their light output control and corresponding energy savings. However, most dimming ballasts are incapable of operating lamps independently. The majority of lamp configurations used with existing dimming ballasts tend to be in series, meaning that if any one lamp is removed all of the accompanying lamps will necessarily be shut down, and further when any individual lamp reaches an end-of-life condition the ballast itself is going to be shut down. These operations result in large expense and inconvenience.
Traditional so-called parallel lamp ballasts provide drawbacks of their own, in that whenever a lamp is removed the lamp current through the remaining lamps in the circuit will change dramatically. A true parallel lamp configuration should maintain the same lamp current regardless of the number of lamps connected to the ballast at a given time.
Lamp ballasts as previously known in the art have sought to provide true parallel operation by providing a decoupled half-bridge inverter configuration, with one line branch (and associated switching element) shared among each discharge lamp and various additional line branches having dedicated switching elements and independently assigned to a particular discharge lamp. These configurations allow for each lamp to be disabled independently, and further operate at the same lamp current output regardless of the failure or removal of one or more lamps.
However, these configurations as previously known have large switching losses due to hard switching of the shared switching element, which renders such applications substantially unusable.